I quilt like this a lot. I do my first wavy line in the middle, but start at the top and stitch to the bottom, edge to edge. Then I work out from there, alternating right and left of the center until I get to the outer edges. I usually only do vertical lines, but if you wNt to add the horizontal ones, I would do it the same---do your first line across the middle, edge to edge, then add the others. Nothing should shift due to all the vertical lines. The risk is puckering when you do the horizontal lines, so just try to ease the fabric as you go.